1. Field of the Invention
This invention relates to a hydrostatic axial piston machine having a swash plate construction. Specifically, the present invention relates to a construction in which the swash plate position can be set by at least one positioning piston pressurized with control pressure, and where a control valve is in a line leading to the positioning piston.
2. Background Information
Axial piston machines are used primarily as hydraulic pumps in hydraulic systems. During operation of these machines, it is advantageous if the displacement volume adapts to different operating conditions by changing the diagonal position of the swash plate. For this purpose, known devices normally use mechanical or hydraulic control devices which are mechanically, hydraulically or electrically actuated.
Known hydraulic control devices have at least one positioning piston engaged with the swash plate to determine the swash plate position. This will determine the displacement volume. A control valve is located in a leading to the positioning piston to generate a control pressure operating the piston.
Electrically actuated control devices have been used to improve the control and regulation of the displacement volume adjustment. Known systems have provided a proportional valve actuated by a proportional magnet.
The proportional magnet converts an electrical control signal into a magnetic force to actuate a pressure reducing valve which moves against a spring force. The pressure reducing valve is connected to a pressure source and generates a pilot pressure as a function of the actuation. The pilot pressure displaces a spring-activated pilot piston. The travel is transmitted by an intermediate mechanical element to a control valve located on the swash plate. The pilot piston actuates the control valve mechanically. The control valve generates a control pressure from a supply pressure. The positioning piston of the hydraulic adjustment device is pressurized with the control pressure. The swash plate position is thereby adjusted. A mechanical linkage transmits the travel back to the control valve to close the valve when the swash plate reaches the desired position.
To generate the pivoting angle of the swash plate by the electrical control signal, the known systems require five conversions in the signal transmission path. Each of these conversions is subject to varying tolerances and requires physical components. Friction occurs in some of the components which is reflected in the form of hysteresis. With the control valve located directly on the swash plate, the lines leading to the positioning piston are more complex and more expensive to construct.
The object of this invention is to make an electric-hydraulic control for the swash plate which has a simple construction.
The above objects can be accomplished by electrical actuation of the control valve according to the present invention. In the present invention, an electrical control signal is converted into a swash plate position with a minimum of intermediate elements.
The electrical control signal directly actuates the control valve which controls the control pressure to the positioning piston. The control valve actuation produces the control pressure which, through the positioning piston, results in the desired diagonal position of the swash plate. With direct actuation of the control valve, the signal transmission path requires only three conversions from the electrical control signal to the desired swash plate position.
In one preferred embodiment, the control valve is actuated by a stepper motor. The electrical signal for the actuation of the stepper motor consists of counting pulses which are converted, independently of friction factors, into the angular displacement of the output shaft of the stepper motor.
The control valve may be a rotary disk valve which pressurizes the positioning piston with control pressure. A rotary disk valve actuated by the stepper motor represents a simple way to generate the control pressure on control edges of the rotary disk valve.
The rotary disk valve may have a rotating control shaft including at least one groove which can be pressurized with supply pressure and with tank pressure. A rotating sleeve surrounds the control shaft on the outside periphery thereof with grooves in the rotary sleeve for the pressurization of the positioning piston with tank pressure or control pressure. Control pressure is exerted on the positioning piston or the positioning piston is pressurized with the tank pressure by changing the angle of rotation of the control shaft relative to the sleeve.
The positioning piston may be a double-acting cylinder or a plurality of single-action cylinders located, for example, on both sides of a swivelling axis of the swash plate. In the first case, the grooves of the sleeve are connected to the piston chamber and the cylinder chamber of the double-acting cylinder. In the latter case, one of each type of groove is connected to the piston chamber of a cylinder. In this embodiment of the rotary disk valve, the function of the control shaft and the sleeve can be exchanged by connecting the grooves in the sleeve to the supply pressure and tank pressure and providing the grooves of the control shaft for the pressurization of the positioning piston.
One of the two rotating components of the rotary disk valve may be non-rotationally connected to the stepper motor output shaft. The electrical input signal is thereby converted directly into an angle of rotation of the rotary disk valve and generates the control pressure for the positioning piston.
The additional rotating component of the rotary disk valve may be engaged with the swash plate. This arrangement creates a correspondence between the displacement of the stepper motor output shaft and the diagonal position of the swash plate in a simple manner. The positioning piston is pressurized with control pressure only as long as there is a difference regarding the angle of rotation of the two components in the rotary disk valve.
The stepper motor output shaft, or the component of the rotary disk valve which is non-rotationally connected to the output shaft of the stepper motor, may be effectively connected to a device which places the output shaft in a neutral position. This guarantees that the stepper motor output shaft and the corresponding component of the rotary disk valve are pulled back into the neutral position, e.g., in the event of a power failure, and then the swash plate will pivot into the neutral position.
Furthermore, the stepper motor output shaft, or the component of the rotary disk valve, which is non-rotationally connected to the stepper motor output shaft, may be connected to a device which monitors the angle of rotation and/or the neutral position of the output shaft. It is thereby possible to monitor the angle of rotation and/or the neutral position of the output shaft if the stepper motor does not convert electrical counting pulses into a rotational movement of the rotary disk valve. It is thereby possible to correct the neutral position in safety routines.
In one embodiment of the invention, a mechanical transmission may be located between the swash plate and the rotational component of the rotary disk valve which is connected to the swash plate. This configuration can change the translation ratio between the angle of rotation of the stepper motor output shaft and the swash plate position. For example, if an increased translation is selected on the mechanical transmission between the swash plate position and the angle of rotation of the rotary disk valve, a desired displacement at the swash plate will be reflected by a correspondingly greater angle of rotation of the rotary disk valve. This arrangement makes possible a rapid adjustment of the swash plate with a high degree of accuracy. The dimensions of the rotary disk valve and the stepper motor may be reduced, if necessary.
In one embodiment, the stepper motor output shaft is non-rotationally connected to the rotational control shaft of the rotary disk valve, and the sleeve of the rotary disk valve is engaged with the swash plate. This configuration provides a simple construction of the axial piston adjustment device.
The rotary disk valve and the stepper motor can be located separately from the swash plate on the housing of the axial piston machine. This can significantly reduce the complexity and cost of the control pressure lines leading to the positioning piston. The longitudinal axis of the rotary disk valve may be perpendicular to the axis of rotation of the axial piston machine and, if necessary, may be aligned with or parallel to the pivoting axis of the swash plate.
In one embodiment of the invention, the stepper motor and the rotary disk valve are oriented parallel to the axis of rotation of the axial piston machine. This arrangement results in a significant reduction of the overall dimensions and in significant reduction of the height of the axial piston machine.
A complete understanding of the invention will be obtained from the following description when taken in connection with the accompanying drawing figures wherein like reference characters identify like parts throughout.